A common silicon-based solar battery has a structure that, for example, includes a reflection preventing film and a light-receiving face electrode through an n+-layer on a top face of a silicon substrate that is a p-type polycrystalline semiconductor and that also includes a back face electrode (hereinafter, simply referred to as “electrode” when these electrodes are not distinguished from each other) through a p+-layer on a bottom face of the silicon substrate. The common silicon-based solar battery is configured to take out electric power through the electrodes, that is generated in a p-n-junction of the semiconductor due to reception of light. The reflection preventing film is a film to increase the light reception efficiency by reducing the surface reflection coefficient maintaining the sufficient visible-light transmission coefficient, and is formed by a thin film such as that of silicon nitride, titanium dioxide, or silicon dioxide.
The reflection preventing film has a high electric resistance value and, therefore, obstructs efficiently taking out the electric power generated in the p-n-junction of the semiconductor. Therefore, the light-receiving face electrode of the solar battery is formed using, for example, a method referred to as “fire-through”. According to this electrode forming method, for example: the reflection preventing film is provided for the n+-layer over the whole top face thereof; thereafter, an electrically conductive paste is applied to the reflection preventing film into a proper shape thereon using, for example, a screen printing method; and the work is applied with a baking process. The electrically conductive paste includes as its main components, for example: silver powder; glass frit (flake-like or powder-like glass pieces acquired by melting a glass material, rapidly cooling the melted glass material, and, thereafter, crushing the cooled glass material when necessary); an organic vehicle; and an organic solvent. During the baking process, the glass component in the electrically conductive paste breaks the reflection preventing film and, therefore, ohmic contact is formed by an electrically conductive component in the electrically conductive paste and the n+-layer therebetween (see, e.g., Patent Document 1). Compared to the case where a portion of the reflection preventing film is removed and the electrode is formed in the portion that has the film removed therefrom, according to the electrode forming method, the process procedure thereof is simplified and no problem arises of misalignment of the film-removed portion and the position to form the electrode with each other.
For this forming procedure of the light-receiving face electrode of a solar battery, various suggestions have conventionally been made for purpose of improving the fire-through property, thereby, improving the ohmic contact, and consequently improving the fill factor (FF) and the energy conversion efficiency. For example, according to one of the suggestions: an element of the fifth group such as phosphorus, vanadium, or bismuth is added to the electrically conductive paste; thereby, the oxidation and reduction actions are facilitated of the glass and silver on the reflection preventing film; and, thereby, the fire-through property is improved (see, e.g., Patent Document 1). According to another of the suggestions: a chloride, a bromide, or a fluoride is added to the electrically conductive paste; thereby, the additive assists the action of breaking the reflection preventing film by glass and silver; and, thereby, the ohmic contact is improved (see, e.g., Patent Document 2). The glass in this case is, for example, borosilicate glass.
For a silver-including paste that includes 85 to 99 [wt %] of silver and 1 to 15 [wt %] of glass, it has been suggested to set the composition of the glass to include 15 to 75 [mol %] of PbO and 5 to 50 [mol %] of SiO2 and to include no B2O3 (see, e.g., Patent Document 3). It is described that the silver-including paste is for use in forming an electrode of a solar battery and that the ohmic contact thereof is improved by using the glass having the above composition.
A thick-film electrically conductive composition has been proposed that is formed by dispersing in an organic solvent: silver powder; an additive including zinc; and glass frit having a softening point in a range from 300 to 600 [° C.] (see, e.g., Patent Document 4). The thick-film electrically conductive composition is for use in forming a light-receiving face electrode of a solar battery, and the electric conductivity and the solder-bonding property thereof are improved by the addition of zinc. For same reason, it is proposed to use an additive including manganese instead of using the additive including zinc. (see e.g., Patent Document 5)